Paper Dyeing Process

ABSTRACT

This specification discloses an improved paper dyeing process in which paper pulp stock is prepared and subjected to a hot dispersion process, adding a sulphur dye after the hot dispersion process, and acidifying the dyed pulp stock to fix the dye. The dye can be added at any suitable time immediately following hot dispersion, and may be added just prior to or during the treatment of the pulp by disperser plates or the like.

FIELD

This invention relates to the dyeing of paper materials, such aspackaging grade paper materials.

BACKGROUND

The production of coloured paper for use in packaging can be achieved byprinting the paper with a coloured ink followed by varnishing to preventink rub-off. While packaging products made from such paper performacceptably, the process costs are high.

The dyeing of paper pulp to obtain fast colours using sulphur-based dyeshas been practised for many years. However, the addition of the dye tothe paper pulp in the pulper during stock preparation, in which the dyeis fixed by the addition of alum, causes colouration of the processwater unless pH is lowered to less than 5, which creates environmentalas well as other process difficulties. In addition, in the case of papermills with multi-ply paper making facilities but only one pulper, suchdyeing processes mean that the addition of the dye to the pulper wouldresult in dyeing of the whole sheet, even if colour is only required forthe outer ply of paper. This results in a significant increase in thecost of dyeing.

SUMMARY

It is an object of the present invention to provide an alternativedyeing process which removes at least the colouration problem outlinedabove.

The invention provides a process for dyeing paper materials, comprisingpreparing pulp stock suitable for a paper making process, subjecting thepulp stock to a hot dispersion process, characterised by the steps ofadding a sulphur dye to the pulp stock after the hot dispersion process,and acidifying the dyed pulp stock to fix the dye.

By addition of the dye to the pulp stock making process after the hotdispersion stage, the contamination of the process water is avoided. Inaddition, the dye can be added to the stock for producing a selectedpaper ply, such as the outer ply, rather than dyeing all plies of thepaper.

The dye can be added at any suitable time immediately following hotdispersion and may be added just prior to or during the treatment of thepulp by disperser plates, or the like.

The pulp stock may be de-watered prior to the hot dispersion step, andis passed through disperser plates, or the like, following hotdispersion. When the pulp stock is passed through the disperser plates,or the like, the pulp stock is diluted to about 5% to 6% stockconcentration and the sulphur dye is added at this stage at an additionrate of about 20% to 30%. The dyed pulp stock is then stored in a stockchest for papermaking, and while in the stock chest, the stock isacidified by the addition of a suitable acid, such as dilute sulphuricacid, to increase the pH level to about 5.5 to 6.0. This results incomplete fixation of the dye in the fibres of the stock.

Since the addition of acid to the sulphur dye results in the formationof hydrogen sulphide gas, the stock chest is preferably covered with alid and the gas extracted for treatment.

In one form, the gas is treated by passing it through another stock tankfull of dyed stock prior to acidification. This results in over 90%adsorption of the hydrogen sulphide gas. The vent gas from the top ofthis tank may be passed through a bed of activated carbon, and thisresults in about 99% removal of the hydrogen sulphide gas.

The pulp from the pulper may be fractionated to produce long fibre stockfor the top ply and short fibre stock for the back ply in a two-plypaper. The long fibre fraction may be de-watered by any suitable means,such as a screw press, to about 25% consistency and given hot dispersionby increasing the stock temperature to around 100° C. by the addition ofsteam, following which the hot stock is passed through disperser platesin a known manner. The pulp stock fraction is at this time diluted toabout 5% stock concentration and the sulphur dye added at this stagefollowing which the stock fraction is stored in a stock chest for papermaking. The stock fraction is then acidified as described above to fixthe dye and the hydrogen gas removed, such as described above. Byoperating in this way, the pulp fraction for the outer ply only is dyedthereby resulting in a significant saving in dye costs.

It has been found that the addition of the dye following hot dispersionand dilution to about 5% to 6,% stock concentration results in a betteryield from dye in terms of colour, and fixing the dye at a higher pHusing a stronger mineral acid, rather than alum, is more economical.

The process described above is suitable for use with any sulphur dye andhas been found to be particularly suitable for use with sulphur blackdye, such as Clariant Diresul black P-FT liquid (CI Sulphur Black 1).

The invention also provides a dyed paper product made by the processdefined above.

In the present specification, the paper making process is not describedin any detail since any paper making process known to persons skilled inthe art may be adopted and modified by a dyeing process embodying thepresent invention.

EXAMPLE

Old corrugated cartons were slushed in a continuous pulper and the pulpwas screened through a 3 mm screen to remove contaminants. The screenedpulp was fractionated through a screen with 0.35 mm slots to get twostreams of pulp. The rejects fraction of long fibre was used to feed thetop wire of a paper machine whereas the accepts fraction (short fibre)was used to feed the bottom wire of the paper machine. The long fibrefraction was dewatered to about 25% consistency in a screw press and thethickened pulp was heated with steam in a hot dispersion unit to around95° C. to melt wax, stickies and the like. The heated stock was mixedwith water and sulphur black dye in the disperser at about 5% stockconcentration. This process broke the waxes and stickies into very smallparticles and mixed the black dye thoroughly with pulp. The sulphurblack dye used was Diresul Black P-FT liquid from Clariant. Thecharacteristics of this dye are as follows:

Colour Index: Sulphur Black 1

Specific Gravity: 1.32 g/cc

Active Concentration: 35%

pH: 12

Various amounts of this black dye were added from 20%-30% on a dry fibrebasis which equals 200 kg dye per ton of fibre to 300 kg dye per ton offibre.

The pulp stock mixed with dye was highly alkaline (pH 10-10.5) and thedye was fixed on the fibre by addition of 50% dilute sulphuric acid topH of 5.8 while the stock was held in a stock chest. The resultinghydrogen sulphide gas was extracted and neutralised as described above.

This pulp was fed to the top ply of two-ply fourdrienier machine runningat 330 m/min with production rate of 11 t/hr. The black colour of thetop side of the paper with varying dosages of black dye was as follows:

Sulphur Black dose L*  0% 67 20% 23.5 25% 22 30% 21

The L* values go down as the black colour becomes darker. These resultsshow there is very significant drop in L* by addition of 20% dye butfurther addition in steps of 5% reduce L* only marginally, although thedye is still fully fixed on the fibre.

The machine wire water did not have black colouration showing completefixation of dye on fibre. This effect can sometimes be very helpful ingrade change of paper on paper machine from one colour to another.

The invention has been described by way of non-limiting example only andmany modifications and variations may be made thereto without departingfrom the spirit and scope of the invention.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

1. A process for dyeing paper materials, comprising preparing pulp stocksuitable for a paper making process, subjecting the pulp stock to a hotdispersion process, characterised by the steps of adding a sulphur dyeto the pulp stock after the hot dispersion process of the pulp stockmaking process, and acidifying the dyed pulp stock to fix the dye. 2.The process of claim 1, wherein the dye is added to the pulp stock justprior to or during treatment of the pulp stock by disperser plates orthe like, following hot dispersion.
 3. The process of claim 1, whereinthe pulp stock is de-watered prior to hot dispersion.
 4. The process ofclaim 1, wherein the hot dispersed pulp stock is diluted to about 5% to6% stock concentrate and the sulphur dye is added at an addition rate ofabout 20% to 30%, following which the pulp stock is stored in a stockchest for paper making where it is acidified to fix the dye.
 5. Theprocess of claim 4, wherein acidification increases the pH of the pulpstock to about 5.5 to 6.0.
 6. The process of claim 5, wherein the dyedpulp stock is acidified by means of a suitable mineral acid, such asdilute sulphuric acid.
 7. The process of claim 4, further comprising thestep of extracting hydrogen sulphide gas from the stock chest fortreatment.
 8. The process of claim 7, wherin the hydrogen sulphide gasis treated by passing the gas through another stock chest prior toacidification to adsorb a substantial portion of the gas.
 9. The processof claim 1, wherein the pulp stock is fractionated to produce long fibrestock for the top ply of a two-ply paper material, and a short fibrestock for the back ply of the two-ply paper material, and only the longfibre stock is subjected to the dyeing process.
 10. A process for dyeingpaper material as described in the Example.
 11. Dyed paper productsproduced by the process as claimed in claim 1.